1. Field of the Invention
This invention relates to a liquid crystalline compound and a liquid crystal composition containing the same, each useful in the fields of devices such as liquid crystal display elements and liquid crystal light-switching elements. More particularly it relates to a novel liquid crystalline compound having an optically active group and a liquid crystal composition containing the same and exhibiting a chiral nematic phase and cholesteric phase.
2. Description of the Related Art
It is well known that the importance of optically active substances contained in liquid crystal compositions used for electrooptical elements utilizing liquid crystal phases, namely, liquid crystal display elements or liquid crystal light-switching elements, consists in the usefulness of the substances as components of compositions exhibiting
(i) cholesteric phase (chiral nematic phase) and
(ii) chiral smectic phase.
It is also known that as to cholesteric phase (Ch phase), it is possible to constitute the phase in compounds exhibiting Ch phase by themselves as main components, and it is also possible to constitute the phase by adding an optically active substance to substances exhibiting a nematic phase, and further, in the latter case, it is not necessary that the added optically active substance exhibit a liquid crystal phase by itself. Further, it is also known that Ch phase is not only utilized for display elements utilizing cholestericnematic phase transition, but also utilized in place of nematic phase in order to prevent occurrence of the so-called reverse domain in TN cell (Twisted Nematic cell). Further, the importance of optically active substances as a component constituting cholesteric liquid crystal compositions in STN mode (a mode of making the twist angle of liquid crystals about 180.degree. to 270.degree.) which is one of the relatively new display modes has been more and more increasing.
On the other hand, electrooptical elements using chiral smectic liquid crystals have recently been noted. This new mode utilizes the ferroelectric properties of liquid crystals and according to this mode, there is a notable improvement in the response rate (see Clark et al, Applied Phys. lett., 36, 899 (1980)). This display mode utilizes chiral smectic phases such as chiral smectic C phase (hereinafter abbreviated as SC* phase). It is known that phases exhibiting ferroelectric properties are not limited to SC* phase, and chiral smectic F, G, H, I phase and the like also exhibit ferroelectric phases. When these ferroelectric liquid crystals are utilized for display elements, liquid crystal materials exhibiting ferroelectric liquid crystal phases within a broad temperature range including room temperature have been desired. At present, however, no single compound satisfying such a requirement has been known; hence liquid crystal compositions obtained by combining some compounds together and satisfying the desired specific features as much as possible have been used.
Chiral smectic liquid crystal compositions, as in the case of Ch phase, may be constituted by using compounds exhibiting chiral smectic phase by themselves as main components, but they may also be constituted by adding optically active substances to compounds exhibiting smectic phase, and in this case, too, it is not necessary that the added optically active substances exhibit liquid crystal phases by themselves.